No prior art search was conducted on the subject matter of this specification in the U.S. Patent Office or in any other search facility.
We are unaware of any prior art that is relevant to the catalyst system taught in this specification. The specific catalyst system taught is a palladium catalyst promoted by tungsten. The catalyst system has highly desirable characteristics in that it is effective in the catalytic oxidation of unburned hydrocarbons and the catalytic reduction of oxides of nitrogen without significant production of ammonia when an internal combustion engine with which it is associated is operated under fuel rich (oxygen deficient) conditions.
The disclosure of the present specification teaches a catalyst formation which we consider to be unique. This unique catalyst formation contains palladium and tungsten. The catalyst system finds utility in the simultaneous control of carbon monoxide, unburned hydrocarbons and oxides of nitrogen under engine operating conditions which are on the fuel rich side of stoichiometric. Palladium is a catalyst material which is considerably less expensive than platinum, which has been known in the past for uses such as the catalyst system disclosed herein. Tungsten, of course, is a base metal and is much less expensive than noble metals such as platinum and rhodium.
A principal object of the present invention is a method of using low cost catalyst system that is capable of treating an automotive exhaust produced by burning a rich fuel mixture in an automotive engine to simultaneously eliminate therefrom unburned hydrocarbons, carbon monoxide and oxides of nitrogen. It is a secondary object of this invention to provide a method of using low cost catalyst system that is capable of also treating automotive exhaust gases generated by burning a fuel lean mixture which will give high conversion rates for the oxidation of unburned hydrocarbons and carbon monoxide to carbon dioxide and water.
As is well known to a skilled artisan, an internal combustion engine normally associated with an automobile will generally operate on both sides of a stoichiometric air/fuel ratio during various modes of engine operation. For example, many engine systems are designed to operate slightly fuel deficient during normal cruising modes of the vehicle. At such time, there is more air present than is required to oxidize the fuel. Therefore, the overall operating mode of the system is oxidizing and the catalyst materials present are operating under oxidizing conditions. In other modes of engine operation, for example, during acceleration periods, internal combustion engines associated with automotive vehicles are normally operated on the rich side of stoichiometry. In this condition, there is more fuel present than air to oxidize the same. In such a case, the overall catalyst system is exposed to reducing conditions because there is not sufficient oxygen available over the catalyst system.
The method of using the catalyst system of the present invention is one which under oxidizing conditions is effective in the catalytic oxidation of unburned hydrocarbons and carbon monoxide, and under reducing conditions is effective not only in the catalytic oxidation of unburned hydrocarbons and carbon monoxide, but also in the catalytic reduction of oxides of nitrogen without significant production of ammonia. The method of using the catalyst system of this specification has these excellent characteristics even though it is using materials substantially less expensive than a material such as platinum, which was previously used for this type of catalyst system.
Another very significant advantage of the method of using the catalyst system of our invention is that the palladium and tungsten materials used are found in the United States. This lowers the dependency of the manufacture of catalyst systems on the availability of catalyst materials from foreign sources.
As mentioned above, platinum generally was the material used in order to achieve the catalytic control of certain materials in exhaust gases. However, platinum has some characteristics which are undesirable in such control systems which are not found with the palladium/tungsten system taught herein. In general, under fuel rich conditions, that is, when the catalyst system is exposed to reducing conditions, the platinum catalyst had good characteristics with respect to the oxidation of unburned hydrocarbons. However, the platinum catalyst does not have good selectivity in the reduction of oxides of nitrogen. By this we mean that the platinum catalyst produces a great amount of ammonia, rather than nitrogen gas, by the reduction of oxides of nitrogen.
The method of using the catalyst system of our invention does not have this side effect under reducing conditions of producing significant quantities of ammonia by the reduction of oxides of nitrogen.